Consider returning NULL for empty dimnames for dfm and fcm

[kbenoit]

Our current behaviour is always to return a character for docnames() and featnames(). This is built into how dimnames() works for fcm and dfm objects. But this is not the default behaviour for base::matrix objects, which return NULL for empty dimnames, and as of Matrix v1.5.5, the base behaviour will be enforced for all Matrix class objects. (see #2258)

library("quanteda")
#> Package version: 4.0.0
#> Unicode version: 14.0
#> ICU version: 71.1
#> Parallel computing: 10 of 10 threads used.
#> See https://quanteda.io for tutorials and examples.

dfmat <- quanteda:::make_null_dfm()
dimnames(dfmat)
#> $docs
#> character(0)
#> 
#> $features
#> character(0)
dimnames(as.matrix(dfmat))
#> $docs
#> NULL
#> 
#> $features
#> NULL

fcmat <- fcm(dfmat)
dimnames(fcmat)
#> $features
#> character(0)
#> 
#> $features
#> character(0)
dimnames(as.matrix(fcmat))
#> $features
#> NULL
#> 
#> $features
#> NULL

^{Created on 2023-05-11 with reprex v2.0.2}

Our options are be one of:

1. Keeping the current behaviour, which was fixed in Fix v3 matrix 1.5.5 #2267 and Fix v4 matrix 1.5.5 #2273, and always return character() for all -- including empty --empty docnames() and featnames(), and for dimnames() on these objects.
2. Keeping the current behaviour for docnames() and featnames() as per (1), but changing dimnames() to return NULL when that function is called.
3. Changing docnames(), featnames(), and dimnames() to return NULL when a dimension is empty. If we choose (3), then we would need to change the docnames() behaviour for all other objects for which docnames() methods are defined.

I would favour (2), and strongly consider (3), but this would need some tests.